Recently, environmental problems on global scale are gathering keen attention. In also the automotive industry, attempts for environmental improvement in the manufacturing steps are positively advanced. Manufacturing steps of automotives give rise to such problems as global warming, generation of industrial waste and discharge of volatile organic compounds (VOC). In particular, most of the VOC is discharged during the coating steps and effective countermeasure is of urgent necessity.
Outer panel portion of automobile bodies are normally coated with multilayer coating film formed of undercoat film of cationic electrocoating, intermediate coat film and top coat film, for imparting corrosion resistance and for aesthetic purpose. With the view to reduce VOC discharge, use of water-based paints for intermediate and top coatings is promoted.
Coating film on automobile bodies is, moreover, required to excel in the film performance, in particular, high finished appearance, chipping resistance (resistance to damages by small rocks sent flying by running cars) and water resistance. In respect of the chipping resistance, the intermediate coating film plays particularly important role.
However, conventional water-based paint in general are inferior in coating film performance such as mechanical properties and water resistance, as compared with organic solvent-based paint. As a means for overcoming this defect, for example, WO 04/61025 discloses a paint composition which contains a copolymer emulsion to which self-crosslinkability is imparted and which has a Tg, acid value and hydroxyl value each falling within a specific range, and a multilayer coating film-forming method using the paint composition, as a water-based intermediate paint composition which, when used to form a multilayer film, gives high chipping resistance and water resistance, and shows good compatibility with top coat and undercoat, and also excellent finished appearance, and a multilayer coating film-forming method using the paint composition. The weight-average molecular weight of the copolymer emulsion used therein is, however, around 50,000-1,000,000 and its effect for improving coating film performance such as chipping resistance is insufficient.
While it is generally effective for improving coating film performance, to use high molecular weight resins (in particular, acrylic resin), there is a problem that use of, for example, intraparticulate crosslinking type high molecular weight acrylic resin emulsion degrades finished appearance such as coated surface smoothness, and coating workability.
On the other hand, JP 2004-137504A discloses aqueous acrylamide polymer solution having a specific concentration and weight-average molecular weight of 500,000-10,000,000 at a specific viscosity range, as an aqueous high molecular weight polymer solution. The main utility of the polymer, however, is paper reinforcing agent. It is unsuitable for use in paint, because it shows unsatisfactory finishing property and coating workability as a high molecular weight acrylic resin for paint use, and coating film formed thereof has insufficient water resistance.